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Can nanoscopic meadows drive electric cars forward?

By Colin Barras

Carbon nanotube “blades of grass” pump an electric current into manganese oxide “nanoflowers”, which can then attract ions out of solution.

(Image: American Chemical Society)

Nanoscale meadows of grass and flowers could hold the key to increasing the amount of energy that can be stored in ultracapacitors, devices tipped to replace batteries in high-demand applications like electric cars.

Batteries are slow to recharge because they store energy chemically. By contrast, capacitors, which are common in electronics, are short-term stores of electrical energy that charge almost instantaneously but hold little energy.

In recent years capacitors able to store thousands of times as much energy as standard ones, called ultracapacitors, have been developed, leading experts to suggest they could power future devices and even electric cars.

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First however, their energy storage capacity needs to improve further. Chinese researchers have just reported a new approach to doing that which could see them become practical.

Ion sponge

Ultracapacitors are simple devices. They are charged by applying a voltage to two electrodes suspended in a solution so that positive ions head to one electrode and negative ions to the other.

Energy is stored because the electrodes are coated with a porous material that soaks up ions like a sponge, usually activated carbon. Improvements in ultracapacitor capacity so far have come from making those carbon sponges with more pores.

Now Hao Zhang at the Research Institute of Chemical Defence in Beijing, China, and colleagues at Peking University have taken a different approach. They store ions in manganese oxide (MnO), a material with a much greater capacity for ions than activated carbon.

However, although MnO holds ions well, it has a high electrical resistance, making it difficult to charge it with voltage to attract ions in the first place.

Double charge

The researchers addressed that by creating a “nanomeadow” of microscopic structures – fuzzy flowers of MnO each about 100 nanometres across on a field of messy carbon nanotube grass grown on a tantalum metal foil (see image, right).

Each flower attaches to at least two of the blades of grass, which act like electron superhighways, says Zhang, forming strong electrical connections to the flowers. The usually resistant MnO can then be charged up to attract the ions it can store so well.

As a result, the nanomeadow performs 10 times better than MnO alone and can store twice as much charge as the carbon-based electrodes in existing ultracapacitors.

Zhang says that the nanomeadow’s complex structure is resistant to the mechanical degradation that reduces the performance of ultracapacitors over time. The energy capacity of the new device drops by just 3% after 20,000 charge and discharge cycles, better than other high-capacity designs.

To the streets

Mike Barnes at the University of Manchester, UK, says this is an interesting approach to improving ultracapacitor performance.

But he points out that that a design ready for market needs to be even more resistant to physical degradation. In vehicles, ultracapacitors are charged during braking, which might happen about 60 times per hour in urban situations.

A delivery van working a five-day, 8-hour week would clock up 120,000 cycles in a year. Going by Zhang’s figures, that would cut its ultracapacitor’s storage ability by at least 15%, something that needs to improve before the nanomeadow design is ready for the road.